FIELD OF THE INVENTION
This invention relates to an anti-lock system for the wheels of a vehicle and more particularly to an anti-lock system for releasing vehicle wheels from a locked state taking place during brake application by reducing the braking hydraulic pressure.
For improved safety of brake application to vehicles, wheel anti-lock techniques of varied kinds have recently been proposed. In a typical example of these techniques, a normally open type hold valve which is normally open and closes when a braking hydraulic pressure must be lowered during anti-lock control is disposed, for example, in a braking hydraulic pressure transmission line (hereinafter referred to as the main line) which is connecting a brake device to a master cylinder (a hydraulic pressure generating device); a by-pass line is connected to the main line in a by-passing manner; the by-pass line is normally closed and is provided with pressure reducing valve which opens when the braking hydraulic pressure must be lowered; braking pressure oil inflowing via the reducing valve into the bypass line is stored within a reservoir device which is arranged, for example, to store oil while lowering hydraulic pressure therein by increasing its interior volume according as pressure is applied thereto; a pressure oil return device which includes a pump is arranged to pump up the stored oil from the inside of the reservoir to the main line; and an electronic control device which detects occurrence of a wheel lock state during brake application to the vehicle is arranged to perform opening and closing switch-over operations on the normally open type hold valve and the normally closed type pressure reducing valve.
However, the pressure oil return device being arranged to include in combination various electromagnetic (or solenoid) valves or reservoir, a pressure accumulator, a pump, etc., the conventional anti-lock control system of the kind described tends to become large in size and high in cost. There has been a great demand for some simpler anti-lock system that is more practically applicable to vehicles. To meet this demand, for example, a so-called two-channel type anti-lock system has been proposed for a vehicle having a hydraulic brake system of a cross-piping type. This system is arranged to have one anti-lock control device for each of intercrossing channels. The system, however, presents the following problem: Generally, the rear wheel brake system of a vehicle is provided with a hydraulic control device which is normally a proportioning valve which reduces or increases the braking hydraulic pressure in comparison with the front wheel side. The output of the proportioning valve (rear wheel braking pressure) has some hysteresis relative to the rise and drop of the input thereof (master cylinder pressure). Compared with a drop in the input, a drop in the output delays. In case that a locking tendency arises only at a rear wheel, the braking hydraulic pressure must be reduced to a sufficiently great degree by the anti-lock control device to remove this tendency. Then, this results in an unnecessarily great degree of reduction in braking hydraulic pressure on the front wheel.